Figure 2.

Speciation models in subterranean taxa. Schematic diagram of (a) the 'climatic relict' and (b) the 'adaptive shift' hypotheses. In the former, a broadly distributed surface species
that has exaptations (pre-adaptations) to the underground environment invades the
caves. The underground population remains in contact with the surface population,
limiting genetic divergence of the two. Climatic oscillations cause local extinction
of surface populations, whereas surviving populations remain in the underground. The
predictions from this for geographic distribution are that either only relict cave-dwelling
lineages survive, or surface populations are strictly allopatric and geographically
remote with respect to the underground. Over time, cave populations differentiate,
developing troglomorphic characters and become reciprocally monophyletic. In the adaptive
shift hypothesis caves are invaded by surface populations, exploiting new resources
with the establishment of differential selection pressures in the epigean and underground
environments. Speciation is driven by divergent selection accompanied by a reduction
of gene flow. In this case, surface and cave species are expected to have parapatric
distributions, at least during the initial phases of the process. Ellipses represent
geographic distributions of populations. Troglobite is the term given to animals that
have become adapted to dwell in cave environments and that cannot survive outside
such environments. Diagram modified from Figures 1 and 2 in [2].